Temsirolimus (CCI-779), an mTOR kinase Inhibitor of formula (I) is an antineoplastic agent indicated for the treatment of advanced renal cell carcinoma. Temsirolimus is a Rapamycin 42 ester with [3-hydroxy-2-(hydroxymethyl)-2-methylpropanoic acid and was first disclosed by Skotnicki et al in U.S. Pat. No. 5,362,718.

Several processes for the preparation of Temsirolimus have been reported in the literature such as those described in U.S. Pat. No. 5,362,718; U.S. Pat. No. 6,277,983 and U.S. Pat. No. 7,153,957.
U.S. Pat. No. 5,362,718 discloses a process for the preparation of different rapamycin 42 esters including Temsirolimus as per the scheme given below (Scheme-I).

The process is non-regioselective and hence results in 31-esterified rapamycin, 31, 42 diesterified rapamycin and unreacted rapamycin along with the desired rapamycin-42 ester.
U.S. Pat. No. 6,277,983 reports a process for the preparation of Temsirolimus by using 31, 42 bis silyl intermediates as per the scheme shown below (Scheme-II).

U.S. Pat. No. 7,153,957 reports a process for the preparation of Temsirolimus by using boronate intermediate as per the scheme shown below (Scheme-III).
